It has been well-known that imparting a swirl to the gas sucked into the combustion chamber of an internal combustion engine will promote the mixing of fuel and air, contributing to improved engine performance by better atomization of fuel and homogenization of the fuel air mixture and an increased speed of flame propagation. Expecially now when the practice of lean burning and high-volume EGR (exhaust gas recirculation) has become widespread for the purposes of exhaust gas purification and fuel saving, swirling the intake gas is found particularly effective.
The conventional means for this purpose include the so-called "helical port" (i.e., an engine suction port designed in a helical configuration which causes the intake gas passing through the gas to swirl as it is introduced into the combustion chamber) or the so-called "eccentric swirl port" (i.e., a suction port which opens into the combustion chamber in a direction tangential to the cylinder wall), so that a swirl may be created in the incoming gas as it flows along the cylinder wall.
In these conventional means, which are intended for high-speed high-load operation, a swirl can be satisfactorily produced by controlling the flow with only the suction port, in the engine work range of medium-speed medium-load to high-speed high-load where the intake gas volume is large; but in the idling and low-speed low-load general working range where the intake gas volume is small and the gas density is low, it is difficult to control the flow of intake gas and accordingly to produce a sufficient swirl in the combustion chamber. Although the form of the helical port may be improved so as to produce a sufficient swirl even in the general working range, in that case the flow resistance will naturally increase to an enormous value, and the volume efficiency will drop, this drop being greater, the higher the engine rpm. For this season said conventional means is virtually inapplicable to the gasoline engine, which runs at an extremely high speed, and its use has been limited to diesel engines, which rotate at a far lower rpm than do gasoline engines.